Electromagnetic induction (eddy-current) flaw detecting method has heretofore been widely used for non-destructive inspection of metal wires and bars that are hot rolled to meet initial prerequisites such as high-speed and non-contact operation. A variety of heat shielding and cooling constructions have been devised for the sensor unit used for the flaw detection since it always confronts the materials of high temperatures that are to be inspected.
In order to protect the sensor unit from high temperatures radiated from the materials to be inspected, heat-resistant coils for eddy-current flaw detection employing a heat-shielding thin-wall sleeve have long been put into practical use as disclosed in Japanese Utility Model Application Publication No. 45-26143, U.S. Pat. No. 3,694,735, Japanese Application Public Disclosure No. 58-83254, and U.S. Pat. No. 4,461,995. In the heat-resistant coils for eddy current flaw detection, the thin-wall sleeve for shielding is made up of a thin-wall pipe composed of an austenite-type stainless steel or titanium resulting, however, in the development of eddy-current loss since it is an electrically conductive material. That is, the magnetic lines of force of the flaw detecting exciting coils or of the detecting coils surrounding the outer circumference of the thin-wall pipe are attenuated by the thin-wall pipe as they reciprocally travel through the hot material to be tested running inside the thin-wall pipe. Attempts were made to replace the thin-wall pipe by a non-metallic pipe such as a ceramic cylinder of alumina or the like without, however, any success of putting it into practice in regard to mechanical resistance and thermal shock resistance.
In order to solve such problems, it was devised to form a single or a plurality of elongated grooves in the heat-shielding sleeve which is made up of a thin metal pipe as disclosed in Japanese Utility Model Application Publication No. 54-26072 and Japanese Patent Application Publication No. 60-18938. With the structure disclosed in Japanese Utility Model Application Publication No. 54-26072, however, the cooling water flows out through the elongated grooves. With the structure disclosed in Japanese Patent Application Publication No. 60-18938, on the other hand, the leakage of water can be prevented by sealing the elongated grooves with a filler material or a junction material. However, since the heat-shielding sleeve and the filler material or the junction material have different coefficients of thermal expansion, the filler material or the junction material is split off causing, after all, the water to leak. There have further been proposed hot eddy-current flaw detecting coils in which the cooling water is positively injected from the heat-shielding member of the hot flaw detecting coil to locally cool the outer surface of the pipe to be inspected after the flaw detecting coil is cooled as disclosed in U.S. Pat. Nos. 4,024,470 and 4,123,708. With the eddy-current flaw detection being carried out while permitting the water to be leaked as done in Japanese Utility Model Application Publication No. 54-26072 and Japanese Patent Application Publication No. 60-18938 and in U.S. Pat. Nos. 4,024,470 and 4,123,708, however, there arises a problem in that the outer surface of the material to be tested is locally cooled.
Therefore, there have been proposed the structures for effecting the cooling without using heat-shielding electrically conductive sleeve which deteriorates the sensitivity of the flaw sensor and without permitting the cooling water to come in contact with the outer surface of the material to be inspected, as disclosed in Japanese Patent Application Public Disclosure No. 61-747. According to the hot flaw detector without sleeve disclosed in Japanese Patent Application Public Disclosure No. 61-747 which uses no heat-shielding metals, the flaw detecting sensitivity in improved by forming a water film along the inner peripheral wall of a non-metallic high molecular coil bobbin such as of nylon or Teflon by using water emitted from the outlet of the coil bobbin, thereby to shield the heat with the water film and to cool. That is, with the structure disclosed in this Japanese Patent Application Public Disclosure No. 61-747 which uses no stainless steel cylinder, the alternating magnetic field for flaw detection is not interrupted by the stainless steel cylinder that was so far used to shield the heat, and the cooling is effected efficiently, too.
With the structure of the hot flaw detector without the sleeve disclosed in the aforementioned Japanese Patent Application Public Disclosure No. 61-747, however, special experience and skill are needed to form a plurality of outlet ports in the coil bobbin maintaining a distance among the ports, an opening width in the axial direction and skews in the water outlet paths with predetermined precision, such that the water emitted from the outlet ports forms a water film along the inner wall and periphery of the cylindrical coil bobbin, spirally whirls toward the exit side, and is drained. If the machining is not well done, ripples develop on the water film whirling along the inner wall of the flaw detecting coil, and the cooling is not effected uniformly.
At the present time in Japan, the hot rolling operation is so carried out as to increase the productivity while giving attention to effecting the low-temperature rolling at near a Curie point and reducing the surface oxidation (scale) of the material to be inspected as a result of low-temperature rolling from the standpoint of saving energy and saving resources. FIG. 5 of the accompanying drawings is a graph showing the working temperatures of carbon steels quoted from the "Handbook of Steels III", from which it will be understood that the hot working region at present is extending up to a region designated by reference symbol A.sub.2. Water-tightening is reinforced so that the roll cooling water will not come in contact with the rolled material in order to save energy, i.e., to prevent the loss of heat. The temperature for heating the material is lowered by the amount of temperature drop that could be prevented by the above countermeasure, contributing to decreasing the fuel cost. Local cooling with water that is positively effected during the hot rolling hinders the operation for saving energy and further impairs the object of gradual cooling after the completion of rolling, and further causes the surface to be hardened and particle sizes to become nonuniform after the smelting. From such a point of view, the structure of the hot flaw detector without sleeve disclosed in the aforementioned Japanese Patent Application Public Disclosure No. 61-747 is not perfect in regard to draining water after the water film is formed to effect the cooling. That is, the water that has whirled along the inner wall of the flaw detecting coil falls due to the force of gravity and comes in contact with the material to be inspected and cools it.